Receipts, securities, and certificates sometimes have special patterns which are printed on the background and contain a text or image visible in a copy so they cannot easily be copied. Such a special pattern is generally called an anti-counterfeit pattern which is designed to prevent easy duplication of an original and serves as a psychological deterrent to copy of the original.
An anti-counterfeit pattern has two regions with the same density: a region where dots remain after copy and a region where dots disappear after copy. The densities of the two regions almost equal. Macroscopically, no hidden image or text such as “copy product” can be perceived. However, the regions have different microscopic characteristics. The hidden text or image will be referred to as a latent image.
For example, the region (called a latent part) where dots remain after copy includes dot blocks with concentrated dots. The region (called a background part) where dots disappear after copy includes dispersed dots. In this way, two regions which have almost the same density and different characteristics can be formed.
The concentrated dots and dispersed dots can be generated by halftoning processing using halftone dots with different screen rulings or dither processing using dither matrices with different features.
A copying machine generally has a limitation of image reproduction capability depending on the input resolution to read tiny dots of a copy document or the output resolution to reproduce tiny dots. If isolated tiny dots beyond the limitation of the image reproduction capability of the copying machine are present in a document, the tiny dots cannot completely be reproduced in a copy product, and the part of the isolated tiny dots drops out.
When the background part of the anti-counterfeit pattern is designed to exceed the limitation of dots reproducible by the copying machine, large dots (concentrated dots) of the anti-counterfeit pattern can be reproduced by copy, although small dots (dispersed dots) cannot be reproduced. Hence, a hidden image (latent image) becomes visible. Even when the dispersed dots do not completely disappear upon copy but obviously have a density difference to the concentrated dots, the hidden image (latent image) also becomes visible.
For the anti-counterfeit pattern, a “camouflage” technique of making a hidden text or image (latent image) more indistinguishable is also known well.
In camouflage, a pattern having a density different from that of the latent part and background part is laid out on the entire anti-counterfeit pattern image. Macroscopically, the camouflage pattern having a density different from that of the latent part and background part is noticeable, and the latent image is more unnoticeable.
An anti-counterfeit pattern containing a camouflage pattern can give a decorative impression to a printed product as compared to an anti-counterfeit pattern without a camouflage pattern.
To easily discriminate the latent image after copy, dots in the camouflage pattern preferably disappear as much as possible after copy. In the simplest implementation, camouflage can be realized by omitting dots in a portion corresponding to the camouflage pattern.
An outline of the anti-counterfeit pattern has been described above (Japanese Patent Laid-Open No. 2001-197297).
Conventionally, printing paper makers print copy-forgery-inhibited-patterns containing a text such as “copy product” or image (latent image) on dedicated paper in advance and sells the paper as anti-copy paper. Government and municipal offices or corporations purchase the anti-copy paper and print documents whose integrity must be guaranteed on the anti-copy paper, thereby deterring copy of the printed product.
The conventional anti-copy paper is produced by the printing paper makers by preprinting copy-forgery-inhibited-patterns on dedicated paper. For this reason, there are disadvantages in terms of cost such as the cost of dedicated paper and the cost accrued by preparing preprinted paper sheets more than necessary.
Recently, however, a technique (called an on-demand copy-forgery-inhibited-pattern output method by a printer) is implemented which creates an anti-counterfeit pattern image by using software and causes a laser printer to output a document with an anti-counterfeit pattern being laid out on the background.
In the on-demand copy-forgery-inhibited-pattern output method by a printer, a document with an anti-counterfeit pattern being laid out on the background can be printed by using normal paper. The document with an anti-counterfeit pattern being laid out on the background can be printed in necessary number at a necessary time. Hence, preprinted paper sheets need not be prepared more than necessary, unlike the prior art. For this reason, in the on-demand copy-forgery-inhibited-pattern output method by a printer, the cost of paper can greatly be reduced as compared to the conventional document copy deterring method using anti-copy paper.
An anti-counterfeit pattern makes it possible to discriminate between a copied product and an original product on the basis of a pattern that appears after copy. Since illicit use of a copied product can be prevented, an effect of psychologically deterring copy can be expected. As described in relation to the prior art, the on-demand copy-forgery-inhibited-pattern output method by a printer is excellent because the anti-counterfeit pattern can be generated easily at a low cost.
In addition, the need for the security of documents is growing more than ever as the electronic documents act comes into effect. The copy-forgery-inhibited-pattern is also required to have measures to strengthen the effects of deterring and tracking unauthorized copy and the reliability of integrity security.